The semipermeable membrane is a membrane which has selective permeability to specified molecules. It is frequently used to remove very small amounts of contaminated molecules dissolved or diffused in a liquid or gas.
In recent years, reverse osmosis has attracted a great deal of interest for utilization in fields involving purification of liquids. This is of special importance when utilizing this system in the purification of water and brackish water. Likewise, the process is also used to remove impurities from liquids such as water or, in the fields of dialysis, blood. When utilizing reverse osmosis in the purification of a brackish water, a pressure in excess of the osmotic pressure of the brackish water feed solution is applied to the solution which is prepared from purified water by a semipermeable membrane. Pure water thereby diffuses through the membrane while the sodium chloride molecules or other impurities which may be present in the water are retained by the membrane.
The efficiency of the reverse osmosis method is greatly affected by the properties of the semipermeable membrane used. Much effort has therefore been made to develop membranes having high performance, and resulted in some specific suggestions.
For example, U.S. Pat. Nos. 3,133,132 and 3,133,137 disclose the early Loeb-type membranes made of cellulose diacetate. These membranes are asymmetric membranes which are characterized by a very thin, dense surface layer or skin that is supported upon an integrally attached, much thicker supporting layer. These known membranes based on cellulose diacetate have the defects of poor compaction, low resistance to chemical and biological degradation, a short useful like, inability of storage in the dry state, and insufficient flux and salt rejection characteristics.
In an attempt to overcome these defects of the Loeb-type membranes, some membranes composed basically of synthetic polymers have recently been suggested. For example, U.S. Pat. No. 3,951,815 discloses a composite semipermeable membrane comprising a microporous substrate and an ultrathin film formed of a crosslinked, grafted polyethylenimide disposed on one surface of said microporous substrate that has been crosslinked with a di- or tri-functional compound such as isophthaloyl chloride and grafted with a graft reactant such as acrylonitrile or epichlorohydrin. U.S. Pat. No. 4,005,012 describes a composite semipermeable membrane comprising an ultrathin film formed by contacting an amine-modified polyepihalohydrin with a polyfunctional agent on a microporous substrate to form this film on one surface of the microporous substrate. Also, U.S. Pat. No. 4,039,440 discloses a reverse osmosis membrane prepared in situ on a porous support by initial formation of a layer of polyethylenimine on the support, followed by interfacial reaction with a polyfunctional reagent to produce a thin surface coating which possesses salt barrier characteristics.
The membrane composed basically of crosslinked polyethylemimine disclosed in U.S. Pat. No. 4,039,440 has a high salt rejection, but has the defect of insufficient water flux and low oxidation resistance (e.g., low resistance to deterioration by the presence of chlorine in the feed saline or brackish water). As one method of improving the oxidation resistance, U.S. Pat. No. 3,951,815 suggests the grafting of acrylonitrile to the polyethylenimine. The acrylonitrile-grafted and crosslinked polyethylenimine shows some improvement in oxidation resistance, but as the membrane is operated continuously for a long period of time, its degradation advances gradually. Moreover, it suffers from the serious defect of having a markedly reduced water flux.
The membrane composed basically of the amine-modified polyepihalohydrin disclosed in U.S. Pat. No. 4,005,012 exhibits a high salt rejection but its water flux is not sufficient. It has been strongly desired to develop membranes having a higher water flux.
The characteristics required of semipermeable membranes are basically high permselectivity and a high flux. In addition, they should have high resistance to compaction, superior resistance to chemical and biological degradation, and sufficient flexibility to endure shaping into modules in actual use such as a tube, spiral or hollow filament. The membranes so far suggested lack one or more of these characteristics, and are not entirely satisfactory for use as semipermeable membranes.
Accordingly, it has been strongly desired in the art to develop membranes having a combination of the aforesaid desired characteristics.